notes-8 - H1) Earthquakes The plates that make up the...

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H1) Earthquakes The plates that make up the earth's lithosphere are constantly in motion. The rate of motion is a few centimetres per year, or approximately 0.1 mm per day (about as fast as your fingernails grow). This does not mean, however, that the rocks present at the places where plates meet (e.g., convergent boundaries and transform faults) are constantly sliding past each other. Under some circumstances they do, but in most cases, particularly in the upper part of the crust, the friction between rocks at a boundary is great enough so that the two plates are locked together. As the plates themselves continue to move, deformation takes place in the rocks close to the locked boundary and strain builds up in the deformed rocks. This strain, or elastic deformation , represents potential energy stored within the rocks in the vicinity of the boundary between two plates. Eventually the strain will become so great that the friction and rock-strength that is preventing movement between the plates will be overcome, the rocks will break and the plates will suddenly slide past each other - producing an earthquake [see Fig. 10.4]. A huge amount of energy will suddenly be released, and will radiate away from the location of the earthquake in the form of deformation waves within the surrounding rock. S-waves (shear waves), and P-waves (compression waves) are known as body waves as they travel through the rock. As soon as this happens, much of the strain that had built up along the fault zone will be released 1 . Earthquakes occur in three main situations: Divergent boundaries Earthquakes at spreading ridges tend to be frequent but small because the rock is too warm to allow significant locking. Transform faults Earthquakes along transform faults are less frequent, but they can be quite large. Most transform faults are beneath the oceans, but some are on land. Subduction zones The deeper parts of subduction zones can have many small earthquakes, but in the shallower parts, where the rocks are cool, earthquakes are less frequent and can be very large. The severity of earthquakes generated in any area will depend primarily on the degree to which the two plates are locked together along the boundary zone. The degree of locking will depend on several factors, including the rock types, the level of water saturation along the boundary, and the temperature. 1 Not all of the strain is necessarily released with the first movement on the rock - especially for large earthquakes. Additional movement - which produces aftershocks - can take place for several days and weeks, and in some cases for months and years.
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Vancouver Island University Geology 111 Discovering Planet Earth Steven Earle 2010 2 The presence of weak or poorly consolidated rocks will prevent a fault zone from becoming locked. The presence of certain minerals, such as clays or graphite will lubricate a fault zone and also reduce the locking tendency. Water will lubricate a fault zone, and thus will reduce the tendency for locking to take place.
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This note was uploaded on 01/13/2012 for the course GLG 111 taught by Professor Staff during the Fall '11 term at Virginia Intermont.

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notes-8 - H1) Earthquakes The plates that make up the...

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